“Cancer-testis” antigens are expressed in several types of tumours but their expression in normal tissues is restricted to testis, which cannot present antigens to T cells because they lack expression of MHC (major histocompatibility complex) class I molecules. Therefore, cancer-testis antigens are considered as tumour-specific antigens, and are potential targets for tumour immunotherapy. Human MAGE-A3 is known to be expressed frequently in a variety of human tumours including melanoma, non-small cell lung carcinoma (NSCLC), bladder cancer, head and neck cancers, squamous oesophageal cancer, and hepatocarcinoma.
MAGE-A3 belongs to the MAGE-A sub-family which comprises 11 known members (MAGEA 1-6 and 8-12). While other “MAGE A” (melanoma antigen family A) genes have been reported (such as MAGE A7, A13-15), the expression patterns of these genes suggest that they are pseudogenes (see e.g., Chomez et al., Cancer Research, 61:5544 (2001)). The 11 MAGE-A genes have their entire coding sequence located in the last exon.
The MAGE-A gene with the highest sequence similarity to the MAGE-A3 gene is MAGE-A6, which is 99% identical, and the differences between the two genes are located within the last exon. The two genes that are the next most closely related to MAGE-A3 are MAGE-A2 and MAGE-A12. At the protein level, MAGE-A6 has 95% sequence identity to MAGE-A3, MAGE-A2 has 84% sequence identity to MAGE-A3, and MAGE-A12 has 85% sequence identity to MAGE-A3. (FIG. 1)
MAGE-N is a new member of the MAGE family, identified from mRNA isolated from a human hepatocellular carcinoma cell line (Wu et al., Chin. J. Cell Mol. Immunol. 18:270-4 (2002)). MAGE-N has been reported to be closely associated with hepatocellular carcinoma (Dong et al., Cancer Biol. Ther. 3(9):891-8 (2004). At the predicted protein level, MAGE-N has <85% sequence similarity to MAGE-A3 and MAGE-A6. MAGE-N is possibly a combination of MAGE-A3, A12, and A1—where the N-terminal region of MAGE-N (amino acids 1-129) has 99% identity to the N-terminal region of MAGE-A3, the central region of MAGE-N differs by one amino acid compared to the central region of MAGE-A12, and the C-terminal region of MAGE-N is identical to the C-terminal region of MAGE-A1.